The objective of this project is to apply quasi-experimental statistical techniques to provide new evidence on the relationship between airborne particulate matter and infant health, as well as document the health benefits of the Clean Air Act Amendments. This research will exploit three quasi-experimental designs. The first quasi-experiment uses sharp, differential air quality changes across counties attributable to geographic variation in the effects of the 1981- 82 recession to estimate the relationship between infant mortality and total suspended particulates (TSPs) air pollution. The second quasi- experiment exploits county-level differences in regulatory stringency (i.e., nonattainment status) introduced by the 1970 Clean Air Act Amendments. It examines whether the regulation-induced reductions in TSPs concentrations in highly regulated counties affected infant mortality rates in the early 1970s. Since the assignment of regulatory intensity is a highly nonlinear function of initial TSPs concentrations, the exercise implements a quasi-experimental regression-discontinuity design. The third quasi-experiment will use data from counties that are monitored for both TSPs and PM10 concentrations in order to determine whether large or small particles are the source of the health effects documented in the first two quasi-experiments. This part of the project also relies on differences in county-level regulatory intensity due to the Clean Air Act Amendments to provide variation in particulates concentrations. The project will utilize the most comprehensive data available on infant health, airborne particulate matter, and environmental regulations. The infant health data comes from the National Mortality Detail Files, National Natality Detail Files, Linked Birth and Infant Death Public-use Data Files, and the Fetal Death Data Public-use Data Files. The airborne particulate matter data was obtained through the EPA and contains annual monitor-level information from the universe of TSPs and PM10 monitors. The regulatory data will come from the annual Code of Federal Regulations that list the counties that are designated nonattainment for TSPs and PM10. This project should add to the stocks of knowledge on the relationship between particulate matter and human health and the benefits of air pollution regulations.